The conventional state of the art for the construction of crowns and bridges for dentistry is largely dependent upon the lost wax technique, which requires the casting of molten metal into an investment material employed to preserve the original form of a wax pattern.
The wax pattern is formed and designed to precisely fit either a die or model which is a replica of the tooth part prepared by the dentist to receive the crown or bridge to be fabricated. The wax pattern is duplicated in an alloy as either a full cast crown, a veneer crown, or a thimble-like coping, pontic, inlay or onlay, to which porcelain may be baked and bonded to form the finished element.
A summary of the state of the art is as follows and as further discussed in the article, "The Ceramic-Metal Restoration", Quintessence of Dental Technology, October 1983, pp. 545-8, A. Milton Bell, D.D.S.
Various ceramic, metallic and ceramo-metal materials have been employed in attempts to improve crowns and bridges. Such materials are disclosed in U.S. Pat. No. 4,243,412, granted Jan. 6, 1981 to Jandon; U.S. Pat. No. 4,249,943 granted Feb. 10, 1981 to Mohammed, et al: U.S. Pat. No. 4,265,669, granted May 5, 1981 to Starling et al; U.S. Pat. No. 2,206,502, granted July 2, 1940 to Heiligman; U.S. Pat. No. 3,649,732, granted Mar. 14, 1972 to Brigham et al; U.S. Pat. No. 4,321,042 granted Mar. 23, 1982 to Scheicher; U.S. Pat. No. 2,106,809 granted Feb. 1, 1938 to Prange, et al; U.S. Pat. No. 3,450,545, granted June 17, 1969 to Ballard et al and U.S. Pat. No. 3,786,565 granted Jan. 22, 1974 to Jarrault. In general;
(1) The dentist prepares a tooth or teeth to be restored by one or more forms of fixed prosthodontic appliances dependent upon the nature of the restoration designed for the particular application (i.e. a crown, bridge, splint or partial denture.
(2) The dentist must then impression the prepared tooth or teeth in an accurate manner in order to permit the accurate duplication of the patient's teeth to be restored by crowns or bridges. With this model or replica the dentist supplies an accurate duplication of the patient's opposing arch and a bite registration in what is known as centric relation. Thus a highly accurate duplication of the prepared teeth and the maxillo-mandibular relationship is prepared in model form on which the prosthesis is to be fabricated. Even in the restoration of a single crown, the dentist must provide the technician with an accurate duplication of the adjacent teeth as well as the opposing teeth in order to permit the building of contact points and occluding contact points in rest position of the jaws as well as in masticatory movements.
(3) Since the final restoration of a crown or bridge must harmonize with the patient's dentition in appearance as well as function, the precise model permits the fabrication of a wax pattern to conform to a specific design for a dental element.
(4) The finished wax pattern is sprued and then removed from the die or model and connected to a sprue former using a precise system of waxes to insure complete cast of the metal in the casting process. Since the wax pattern is removed from the die or model to be invested, this is considered to be an indirect fabrication technique. It is important to note this because discrepancies may be introduced in an indirect technique due to distortion of the wax pattern in the removal from the die. The wax may distort in the investing process, and the wax and investment materials undergo contraction and expansion changes due to temperature changes during burn-out and casting and solidification of the molten metal during the cooling cycle.
(5) The sprued wax pattern is then invested in a gypsum type of material such as cristobalite or a phosphate-bonded high heat material, depending upon the type of metal being cast.
(6) The invested was pattern when set is placed into a burnout oven for a period of one and a half hours or more, depending upon the technique and metal being cast, and the manufacturers instructions of the particular investment used. Temperatures of the burn-out oven may range from 900.degree. F. to 1600.degree. F., and may involve one or more heat stages to insure maximum expansion of the investment to compensate for initial contraction during the setting of the investment. This expansion during the heating cycle varies and may be a cause for an improper fit of the final restoration, if not properly closely controlled. The burn-out procedure not only expands the investment in preparation for the casting of the molten metal, but is essential for the elimination of the wax thus leaving a void in the investment material or a mold of the eliminated wax pattern.
(7) It is customary to use a casting ring to contain the investment material around the wax pattern. Spacers are used to permit the expansion of the investment in the heating stage. This is an imprecise technique and can cause improper fit of the final casting. Some investment materials use a plastic or paper ring for the purpose of forming and containing the investment material for the casting procedure. The plastic or paper is burned off during the burn-out stage or removed after the investment has set. This allows for maximum expansion of the investment during the heating stage. The actual casting is done by placing the investment which was formed by some kind of device, or in a steel ring, into some type of casting apparatus after the burn-out stage. This permits the melting of the desired metal at the required temperature, and the molten metal is then forced into the mold in the hot investment either by centrifugal force, pressure or vacuum. Conventional dental ovens provide for maximum vacuum pressures of 26-29in. Hg, ie. about 60,000 microns Hg. These ovens were typically used as porcelain firing furnaces. There are various types of equipment for these different methods of making a casting. Once the cast has been completed, the metal and investment material must be allowed to cool.
(8) The casting can be recovered from the investment material by breaking out the casting from the investment. The casting is then cleaned of any remnants of the investment material.
(9) The sprues must then be cut off the crown, bridge or pontic and smoothed down. The casting must then be fitted back upon the original die. If the technique employed by the technician utilizes a gypsum die it may be difficult to seat the casting on the model without scraping or chipping the die. The ultimate fit on the tooth is therefore complicated for the dentist.
Miscasts and incomplete castings which fail to reproduce fine margins or parts of the original wax pattern, or poor fit of the casting due to shrinkage and expansion factors of the wax and investment materials is not uncommon. This may require repeating the entire procedure if the casting cannot be properly seated on the die or prepared tooth of the patient.
While dental bridges can either be cast in one piece or assembled from individual units, a more accurate fit is assured by assembly of the units of the bridge or splint from an index impression taken of the units seated in the mouth, which may insure complete placement of the castings upon the individual prepared teeth. This technique has been widely employed for many years, utilizing the precious alloys which are relatively simple to solder. The non-precious alloys employed today are more difficult to solder or braze due to their formation of high oxide layers on their surfaces when subjected to high temperatures during this process. This has led the dentist to prescribe having their technicians cast multiple unit bridges and splints in one piece to eliminate the necessity for soldering. There is some question as to the accuracy of fit of such long space prostheses being cast as one piece.
These long, labor intensive prior art techniques are costly as well as time consuming, and sometimes provide a questionable or inaccurate fit.
Now there is provided by the present invention a direct method for the preparation of crowns, which avoids many of the aforesaid prior art problems.
It is a principal object of the present invention to provide a direct method and resultant prosthetic product having a density equal to or greater than the indirect prior art cast prosthetic.
It is another principal object of the present invention to provide an apparatus for rapidly and accurately achieving the immedatly aforesaid object.
These objects, as well as other objects, will become apparent from the reading of the following description, the adjoined claim, and the accompanying drawings, in which: